The application of number of in vitro parameters of cellular reactivity to the study of methyl-cholanthrene induced fibrosarcoma in Balb/C mice has permitted us to study the relative roles of cellular and humoral mechanisms in this tumor. Our studies have demonstrated that cellular reactivity to tumor antigens does occur in the lymphocytes obtained from these animals, but this reactivity is no longer demonstrable 5-6 days after the intraperitoneal of these tumor cells. Perhaps the most striking finding has been the discovery of a "blocking and/or enhancing" factor(s) in the intraperitoneal fluid of tumor bearing animals which completely inhibits the cytotoxic effect of lymphocytes obtained from these animals. In view of these findings it is planned to further delineate the nature of these blocking factors. Specifically we would like to determine whether this factor inhibits the cytotoxicity of the lymphocyte directly or whether this factor acts by coating the tumor cells, thereby protecting tumor cells from cytotoxic lymphocytes. In addition, it is planned to study the effect of this factor on the cellular recognition of tumor antigens by sensitized mouse lymphocytes. The appearance of a factor in these mice which actively blocks the cellular reactivity of lymphocytes for tumor cells in tumor bearing animals has important implications in the field of tumor immunology. The isolation of this factor and the mechanisms whereby it exerts its action may provide valuable information regarding tumor growth in man and open up possible pathways to overcome the loss of cellular reactivity to tumor antigens.